1. Field of the Invention
The present invention relates to a premixing apparatus for mixing a fuel gas with air to supply a fuel-air mixture through a fan to a burner.
2. Background of the Related Art
As this kind of premixing apparatus, there is known the following art in Tokkyo Kohyo Koho (National Publication of Translated Version) No. 2014-502719. In the known premixing apparatus, a downstream side of a gas feed passage, in which is interposed a flow control valve for supplying a fuel gas, is connected to an air feed passage on an upstream side of the fan. The premixing apparatus comprises: an air resistance changeover means for changing over, between high and low, a flow resistance in the air feed passage; and a gas resistance changeover means for changing over, between high and low, a flow resistance in that part of the gas feed passage which is on a downstream side of the flow control valve (note: the expression of “that part of . . . which is . . . ” means “such a part of . . . as is . . . ”; namely, the last part of the above sentence partially means “a flow resistance in such a part of the gas feed passage as is on a downstream side of the flow control valve”).
By the way, when a proportional valve is used as a flow control valve, control is made of the proportional valve such that the fuel gas depending on the required amount of combustion can be supplied. Further, control is made of the number of fan revolution depending on the required amount of combustion such that a fuel-air ratio of a fuel-air mixture to be supplied to the burner becomes constant. However, in case the required amount of combustion falls below a predetermined amount and, as a result, the number of fan revolution falls below a lower limit value of fan revolution at which the proportional characteristics of the air feed amount can be maintained, or in case the proportional valve current (electric current charge to the proportional valve) falls below the lower-limit current at which the proportional characteristics of the gas feed amount can be maintained, the air or fuel gas depending on the required amount of combustion can no longer be supplied.
Further, there is a case in which, as the flow control valve, a zero governor is used to maintain the secondary gas pressure at the atmospheric pressure. In this case, the gas feed amount varies with the differential pressure between the atmospheric pressure which is the secondary gas pressure and the negative pressure in the air feed passage. And since the negative pressure in the air feed passage varies with the number of fan revolution, the fuel gas feed amount varies with the number of fan revolution, i.e., the air feed amount. Therefore, by controlling the number of fan revolution depending on the required amount of combustion, the amount of air and fuel gas depending on the required amount of combustion will be supplied to the burner.
In this arrangement, too, when the number of fan revolution falls below the lower-limit revolution at which the proportional characteristics of the air feed amount can be maintained, the amount of air or fuel gas depending on the required amount of combustion can no longer be supplied. Therefore, when the required amount of combustion has fallen below a predetermined amount, the following becomes necessary. In other words, by increasing the flow resistance in the air feed passage by means of the air resistance changeover means without decreasing the number of fan revolution below the above-described lower limit value, the air depending on the amount of required combustion below the predetermined value must be supplied. In addition, only by increasing the flow resistance in the air feed passage will result in that, due to an increase in the negative pressure in the air feed passage, the fuel gas feed amount will exceed the amount depending on the required combustion amount. It is therefore necessary to increase also the flow resistance in the gas feed passage to suit an increase in the flow resistance in the air feed passage.
To comply with the above, the above-described known art has the following arrangement. In other words, when the required combustion amount has fallen below a predetermined value, the flow resistance in the air feed passage can be increased by the air resistance changeover means and, at the same time, the flow resistance in the gas feed passage is increased by the gas resistance changeover means so that air or fuel gas in an amount depending on the required amount of combustion below the predetermined value can be supplied.
In the above-described known art, the following arrangement is employed. In other words, the air feed passage is once branched into two, i.e., the first and the second, air passages on the way and they are merged. The merged portion is provided with a valve seat having formed therein an opening which allows for flow of air from the first air flow passage. Air resistance changeover means is constituted by a valve which closes the opening by seating on the valve seat. It is thus so arranged that, by closing the opening, the flow resistance in the air feed passage becomes larger. In addition, a downstream end of the gas feed passage is branched into two branch passages connected to the second air feed passage. A valve that opens and closes the connection port of one of the branched passages relative to the air feed passage constitutes the gas resistance changeover means. It is thus so arranged that, by closing the connection port, the flow resistance in the gas feed passage increases.
This arrangement, however, has a problem in that, in case the flow resistance in the air feed passage is increased, the air is caused to flow only in the second air feed passage and, therefore, that the negative pressure is not generated so much. As a result, the suction force of the fuel gas becomes weaker, so that stable gas introduction becomes difficult. In addition, in case the air flows also in the first air feed passage to thereby decrease the flow resistance in the air feed passage, the fuel gas is introduced into the second air flow passage. As a result, the fuel gas does not successfully get mixed with the air that has been introduced into the first air flow passage, resulting in an uneven distribution of fuel-air ratio.